Understanding Seismic Activity During the 2024 Forge Circulation Test

The Utah FORGE (Frontier Observatory for Research in Geothermal Energy) recently conducted a test to determine how well fluids can circulate between a newly developed pair of wells. Fluid was injected into the injection well, flowed through previously fractured hot dry rocks, and then traveled up the production well at a much higher temperature. The fluid flow and temperature at the production well indicate energy generation potential.

During geothermal production, small seismic events (earthquakes) occur deep underground. Most of these events are very small and go unnoticed. Rarely, however, moderately sized events can occur and cause shaking on the surface. To help manage these risks, FORGE implements a comprehensive Traffic Light System (TLS). This post explains the TLS and what we know so far about seismicity related to the FORGE circulation test.

Traffic Light System

The TLS outlines responses that are taken when signs of potential seismic issues (triggers) are detected. There are different TLS for circulation/stimulation conditions versus production conditions. Additionally, the TLS make a distinction between events occurring on the FORGE site versus at neighboring geothermal sites. For simplicity, this section only describes the TLS for events occurring near the FORGE site during a circulation test.

The triggers for the TLS include:

  • Normal: No anomalous activity (defined below) detected.
  • 10M≥1(3km): 10 or more events with magnitude 1 or greater located within 3 km of the reservoir in a 24-hour period.
  • Fault plane: Events appear to propagate along an imaged fault plane.
  • Stimulation zone: Events cluster in a single stimulation zone.
  • M≥2(3km): An event with a magnitude 2 or greater occurred within 3 km of the reservoir.
  • M≥3(5km): An event with a magnitude 3 or greater occurred within 5 km of the reservoir.
  • M≥3(5-15km): An event with a magnitude 3 or greater occurred between 5 and 15 km of the reservoir. It is unlikely the event is related to FORGE but merits careful observation.

The responses for the TLS are:

  • No action: Continue operation as planned.
  • Notify WSM: Inform the Well Site Manager (WSM) of the activity. The WSM will inform other entities as needed.
  • Reduce injection: Decrease fluid injection volume to lower reservoir pressure.
  • Flowback: Allow fluid to flow back up the injection well to gradually reduce reservoir pressure.
  • Open shut-ins: Open all nearby shut-in wells (excluding the production and injection wells) to allow additional pressure release.
  • Shutdown: Cease all operations until the appropriate parties are contacted and a plan is developed.
  • Secure well: When safe to do so, disassemble all equipment used for injection.
Trigger Response
Normal No action.
M≥3(5-15km) Notify WSM
10M≥1(3km) Notify WSM.
10M≥1(3km) and stimulation zone Open shut-ins, reduce injection by 10%. If more M≥1(3km) events occur, continue to reduce injection by 10% increments. When green conditions return continue with operational plan.
10M≥1(3km) and fault plane Open shut-ins, flowback
M≥2(3km) Notify WSM, shutdown
M≥3(5km) Notify WSM, flowback, secure well

Observations

During the test, 2,817 events were detected and located with most events having a magnitude less than 1 (Figure 1). There was one magnitude 2 event that triggered a yellow status on April 7, and a magnitude 3.1 that triggered a red alert on April 15.

Although the investigation into the magnitude 3.1 event is underway, it is important to keep in mind that this earthquake is still quite small and unlikely to cause damage. For example, the United States Geological Survey indicates earthquakes below magnitude 4 or 5 rarely cause damage1 and, for comparison, California regularly experiences over 50 earthquakes above magnitude 3.0 each month 2.

Figure 1: Seismic events and magnitudes associated with the circulation test. The Vertical lines indicate the times TLS alerts were issued.

Examining the event locations (Figure 2), earthquakes tend to cluster along two distinct zones elongated in the north–south direction. This may indicate fluid movement along preexisting joints or faults oriented in that direction.

Figure 2: Seismic events (dots) sized according to magnitude and colored by depth. Also shown are the wells (gray lines) and permit boundaries (green line).

The following figure provides an interactive view of the events with magnitude >= -0.5 in relation to the surface, the granitoid (the igneous rock where the heat is stored), and the wells.

Why This Matters

Induced seismicity can raise concerns. That’s why the Utah FORGE team takes proactive steps:

  • Real-time monitoring ensures a swift operational response.

  • Transparency with regulators and the public helps maintain trust.

  • Data-informed modeling supports safer future geothermal developments.

This test demonstrates that well-designed experiments, supported by real-time monitoring, help advance geothermal technology without compromising public safety.